gitweb: Check permissions first in git_search

Check first if relevant features: 'search', 'pickaxe', 'grep', as
appropriate, are enabled before doing anything else in git_search.
This should make git_search code more clear.

While at it, expand a bit error message (e.g. 'Pickaxe' ->
'Pickaxe search').

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>